It has also
been observed that when the pollen of one species is placed on the stigma
of a distantly allied species, though the pollen-tubes protrude, they do
not penetrate the stigmatic surface. Again, the male element may reach the
female element, but be incapable of causing an embryo to be developed, as
seems to have been the case with some of Thuret's experiments on Fuci. No
explanation can be given of these facts, any more than why certain trees
cannot be grafted on others. Lastly, an embryo may be developed, and then
perish at an early period. This latter alternative has not been
sufficiently attended to; but I believe, from observations communicated to
me by Mr. Hewitt, who has had great experience in hybridising pheasants and
fowls, that the early death of the embryo is a very frequent cause of
sterility in first crosses. Mr. Salter has recently given the results of
an examination of about 500 eggs produced from various crosses between
three species of Gallus and their hybrids; the majority of these eggs had
been fertilised; and in the majority of the fertilised eggs, the embryos
had either been partially developed and had then perished, or had become
nearly mature, but the young chickens had been unable to break through the
shell. Of the chickens which were born, more than four-fifths died within
the first few days, or at latest weeks, "without any obvious cause,
apparently from mere inability to live;" so that from the 500 eggs only
twelve chickens were reared.
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